As is well known, under small-signal conditions, the output current iOUT of a transistor is proportional to the input voltages vin applied thereto by a proportionality coefficient called transconductance gm.
This transconductance generally varies with temperature and process parameters as a function of the chosen transistor type and of the particular integration process. Therefore, different types of transistors have transconductances that are typically uncorrelated to each other and that are variable with respect to temperature according to convenient laws.
For example, a bipolar NPN transistor and a NMOS transistor are shown with reference to FIG. 1, whose input-output relation under low-signal conditions are respectively:ic=gmBJT·Vin1  (1)id=gmMOS·Vin2  (2)the respective transconductance expressions are:
                              g          mBJT                ≡                              α            ·                          I              E                                            v            T                                              (        3        )                                          g          mMOS                =                  2          ⁢                                                    K                n                            ·                              W                L                            ·                              I                D                                                                        (        4        )            where                α is the current gain of the common-base-configured bipolar transistor        vT is the thermal voltage        W/L is the form factor of MOS transistors        Kn is a process constant of the NMOS transistor being proportional to the mobility μn and to the oxide specific capacity Cox         IE is the emitter bias current of the bipolar transistor Q1        ID is the bias current of the MOS transistor M1        
From a comparison between the expressions (3) and (4) it is evident that the transconductances of the two types of transistors depend on different process parameters and they also have different laws of variation with temperature and they are thus generally uncorrelated.
At present, there is no effective solution for correlating the transconductance values of different types of transistors, i.e., to achieve a relation like the following one being mediated by a proportionality coefficient γ.Gm—TYPE1=γ·gm—TYPE2